1. Field of the Invention
The present invention relates to leakage diagnostic devices for fuel vapor processing apparatus.
2. Description of the Related Art
Known vehicles that run on gasoline or like fuel have a fuel vapor processing apparatus that can prevent fuel vapor from being dissipated to the atmosphere while preventing damage on a fuel tank due to increase of an internal pressure of the fuel tank. However, if the fuel vapor processing apparatus is cracked or has an improper seal portion, it may be possible that fuel vapor may leak from the processing system. An operator cannot directly recognize occurrence of such leakage of fuel, vapor. Therefore, a diagnostic device for diagnosing occurrence of leakage from a fuel vapor processing device has been proposed, for example, in Japanese Laid-Open Patent Publication No. 6-235354.
The fuel vapor processing apparatus disclosed in the above publication incorporates an evaporation purge system communicating between a canister and an intake air passage of an engine for desorbing fuel vapor from the canister by a negative pressure produced as an intake air is introduced into the engine, and for purging the desorbed fuel vapor into the engine. On this condition, the diagnostic device diagnoses leakage by applying a negative pressure produced by the intake air to the entire process system including a fuel tank. That is, the intake air passage (intake air) is used as means for desorbing the fuel vapor and for applying the pressure. The leakage diagnosis is made, based on a result of detection of a pressure by an internal pressure sensor (pressure detecting device), from a pressure reducing ratio within the process system due to the application of the negative pressure or a pressure increasing ratio resulting when. the process system is closed under the negative pressure condition. However, the pressure of the fuel vapor may influence the internal pressure within the process system. Therefore, in order to avoid wrong diagnosis that may be caused by change of the fuel vapor pressure, a second embodiment disclosed in the above publication has proposed to correct a diagnosis reference pressure for leakage based on an amount of production of fuel vapor (or a fuel vapor pressure), which is predicted from a temperature of the fuel within a fuel tank.
More specifically, when a negative pressure is applied to the process system, as indicated by a solid line in FIG. 13, an internal pressure P (measured value) within the process system gradually decreases and then gradually increases (i.e., returns to the side of the atmospheric pressure) due to generation of the fuel vapor again even if the process system is closed under the negative condition to maintain a negative pressure. If the internal pressure P at diagnosis timing T1 during application of the negative pressure or that at diagnosis timing T2 during maintaining the negative pressure is less than a reference pressure Pm that is a reference for diagnosis of leakage, it is determined that no leakage occurs. On the other hand, if leakage occurs from the fuel vapor processing apparatus, an external air may flow into the process system. Therefore, the internal pressure P gently reduces during the application of the negative pressure as indicated by a one-dot chain line in FIG. 13. During maintaining the negative pressure, the internal pressure P rapidly increases as indicated by a two-dot chain line in FIG. 13. Therefore, at the diagnosis timing T1 during application of the negative pressure, the internal pressure P is lower than the reference pressure Pm, while at the predetermined diagnosis timing T2 during maintaining the negative pressure, the internal pressure P is higher than the reference pressure Pm. By using this, it is determined that leakage occurs if the internal pressure P of the process system during application of the negative pressure or during maintaining the negative pressure is higher than the reference pressure Pm.
However, this leakage diagnosis technique using application of the negative pressure may not provide a correct diagnosis result depending on the fuel temperature or the vaporization condition of the fuel in the process system. The fuel temperature may increase due to the external temperature or heat produced during driving of a fuel pump that pumps the fuel to be supplied to an engine. If the fuel temperature increases, the fuel may be easily vaporized to cause increase of the fuel vapor pressure as shown in FIG. 14. This may cause increase of the internal pressure within the process system. In such a case, as indicated by a dashed line (broken line) in FIG. 13, the internal pressure P varies while maintaining a high pressure level. Therefore, it may be possible that the internal pressure P at the diagnosis timings T1 or T2 becomes higher than the reference pressure P even during application of the negative pressure. Therefore, even in the case that no leakage occurs from the fuel vapor processing apparatus, a wrong diagnosis is made to determine that leakage occurs. To this end, according to the second embodiment of the above publication, the reference pressure Pm is corrected by predicting an amount of production of fuel vapor (or a fuel vapor pressure) from a temperature of the fuel within the fuel tank.
The same problem as described above may be caused also in the case that leakage diagnosis is made by applying a positive pressure to the process system. Thus, in the case that a positive pressure is applied to the process system, the internal pressure P gradually increases as indicated by a solid line in FIG. 15. If the internal pressure P at diagnosis timing T1 during application of the positive pressure or that at diagnosis timing T2 during maintaining the positive pressure is higher than a reference pressure Pp, it may be determined that no leakage occurs. If leakage occurs from the fuel vapor processing apparatus, gas within the process system may flow out of the process system. Therefore, the internal pressure P gently increases during the application of the positive pressure as indicated by a one-dot chain line in FIG. 15. During maintaining the positive pressure, the internal pressure P is lowered as indicated by a two-dot chain line in FIG. 15. Therefore, at the diagnosis timing T1 during application of the positive pressure, the internal pressure P is lower than the reference pressure Pp, and also at the predetermined diagnosis timing T2 during maintaining the positive pressure, the internal pressure P is lower than the reference pressure Pp. By using this, it is determined that leakage occurs if the internal pressure P of the process system during application of the positive pressure or during maintaining the positive pressure is lower than the reference pressure Pp.
However, also in the case of leakage diagnosis made by applying the positive pressure, because the fuel vapor pressure increases as the fuel temperature increases, the internal pressure P within the process system varies while maintaining a high pressure level as indicated by a dashed line (broken line) in FIG. 15. Therefore, even in the case that leakage occurs from the fuel vapor processing apparatus, it may be possible that the internal pressure P at the diagnosis timings T1 becomes higher than the reference pressure Pp although the internal pressure P gently increases during application of the positive pressure. In addition, even in the case that leakage occurs from the fuel vapor processing apparatus, it may be possible that the internal pressure P at the diagnosis timings T2 becomes higher than the reference pressure Pp although the internal pressure P decreases during application of the positive pressure. In this way, it may be possible that the internal pressure P is higher than the reference pressure Pp at the timings T1 and T2 in the case that the positive pressure is applied when the fuel temperature is high. Thus, there is also a problem that a wrong diagnosis is made to determine that no leakage occurs even in the case that leakage occurs.
Incidentally, Japanese Laid-Open Patent Publication No. 2002-235608 proposes a fuel, vapor processing apparatus incorporating an aspirator that can produce a negative pressure by utilizing a part of fuel discharged from a fuel pump without using the intake air passage (negative pressure of intake air). According to the fuel vapor processing apparatus of this publication, the aspirator communicates with the fuel pump via a pressure regulator for regulating the pressure of the fuel, while a decompression chamber of the aspirator communicates with a canister. Therefore, a negative pressure produced by introducing surplus fuel from the pressure regulator into the aspirator is applied to the canister, and the fuel vapor within the canister is recovered into the fuel tank via the aspirator. Thus, the fuel vapor processing apparatus of this reference incorporates a purge-less evaporation system in which the fuel vapor is recovered into the fuel tank without being purged into the intake air passage.
As described above, according the technique of Japanese Laid-Open Patent Publication No. 6-235354, the reference for diagnosis of leakage is corrected by predicting the fuel vapor pressure from the fuel temperature. However, although the fuel vapor pressure has a correlation with the fuel temperature, fuel, such as gasoline, is a mixture of various kinds of hydrocarbons. Therefore, as shown in FIG. 14, the fuel vapor pressure has a characteristic that may vary according to the kind (composition) of fuel. In general, lighter fuel has a higher fuel vapor pressure, while heavier fuel has a lower fuel vapor pressure. The pressure characteristic also may vary according to the storing condition (degradation condition) of the fuel. In the technique of the Publication No. 6-235354, the fuel vapor pressure is predicted only from the fuel temperature, and no actual temperature is measured. Therefore, it is likely that there is a difference between the fuel vapor pressure predicted from the fuel temperature and the actual fuel vapor pressure of the fuel that is being used. This does not allow to accurately correct the reference for diagnosis, leading to a wrong diagnosis of leakage.
In the case of Japanese Laid-Open Patent Publication No. 2002-235608, the fuel, vapor is recovered from the canister by using the aspirator. However this publication does not disclose a technique of diagnosing leakage from the fuel vapor processing apparatus.
Therefore, there is a need in the art for a leakage diagnosis device that can accurately determine whether or not leakage occurs regardless of change of the kind of fuel.